Web Push Notifications work on both desktops and mobile devices. You may not find the opt-in prompt if you are browsing on an unsupported device or browser.
Apple doesn't support web push notifications on iOS devices yet. Until Apple themselves release a support for this, no web push app can send notifications to iOS devices ( iPhones/ iPads).
Since Apple doesn't support web push notifications on iOS devices (iPhones, iPad), we do not show price drop and back in stock widget on iOS devices.